The present invention relates to a display apparatus and in particular to a circuit for correcting pincushion distortion and a display device comprising the circuit.
In a cathode ray tube (termed xe2x80x9cCRTxe2x80x9d) of a Television receiver or display apparatus, as shown in FIG. 16a, while a deflection radius of a uniform magnetic field for deflecting an electron beam 73 emitted from an electron gun (cathode electrode 72) defines a spherical face, a screen (fluorescent surface) 74 is substantially planer so that the distance from the deflection center to the screen gets longer with the increase of the deflection angle and the projection size becomes larger as a raster image projected on the screen 74 moves toward the edges of the screen 74. The raster image are extended most at the four corners where the deflection angle becomes the largest and are distorted in the form of a pincushion and hence this distortion is termed xe2x80x9cpincushion distortionxe2x80x9d. As shown in FIG. 16b, the distortion in which the vertical line at opposing right and left edges on the screen are curved is referred to as xe2x80x9cE-W Raster pincushion distortionxe2x80x9d or xe2x80x9cSide pincushion distortionxe2x80x9d since it is the distortion at the right and left edges.
Correction of the left and right pincushion distortion has heretofore been achieved by causing a horizontal deflection current which is amplitude-modulated with a parabolic current in a vertical scanning period to flow through a horizontal deflection coil.
FIG. 17 is a block diagram showing an exemplary configuration of a CRT monitor in which a deflection distortion such as a side pincushion distortion is corrected. Referring to FIG. 17, the CRT monitor comprises a vertical sawtooth wave oscillating circuit 207 which receives a vertical synchronization signal Vsync output from a synchronization separating/synchronization input processing circuit 205 and generates a vertical sweeping sawtooth wave (termed vertical sawtooth wave) and a deflection distortion correcting circuit 208 which receives the vertical sawtooth wave output from the vertical sawtooth oscillating circuit 207 and generates a distortion correction signal Vo made up of a parabolic wave of one vertical sweeping period. The distortion correction signal Vo which is generated by the deflection distortion correcting circuit 208 is fed to a horizontal oscillating circuit 206 which receives a horizontal synchronization signal Hsync and generates a horizontal sweeping sawtooth wave (horizontal drive signal). The horizontal drive signal (the phase of which is controlled by the distortion correction signal) which is output from the horizontal oscillating circuit 206 is fed to a horizontal deflection output circuit 210. The horizontal drive signal output from the horizontal oscillation circuit 206 and the distortion correction signal Vo output from the deflection distortion correction circuit 208 are fed to a +B power supply circuit 209 which boosts the +B voltage by a pulse amplification in a blanking period during which a high voltage is required. As is well known, one cycle of the sawtooth wave is made up of a scanning interval during which the electron beam is scanned at a constant rate and a blanking interval which is a period from the completion of the scanning in one direction to next scanning. The amplitude of the horizontal drive signal within one vertical period is corrected by correcting +B voltage with the distortion correction signal Vo, a frequency of which corresponds to one vertical period. The +B voltage is supplied by the +B power supply circuit 209 to the horizontal deflection output circuit 210 which provides a deflection current which flows into a horizontal deflection coil. The horizontal deflecting current in which the amplitude of the horizontal drive signal is changed in proportion with that of the distortion correction signal is output from the horizontal deflection output circuit 210. The synchronization deflection circuit is configured as mentioned above. As an output circuit of a video signal (RGB signal), there exists an RGB preamplifier 201 which receives a input video signal (RGB input) and outputs pre-amplified signal to an RGB main amplifier 202. The amplified output signal from the RGB main amplifier 202 is fed to an electron gun (not shown) of the CRT 203. In the CRT 203, deflection currents from horizontal and vertical deflection output circuits 210 and 211 are supplied to the horizontal and vertical deflection coils 204, respectively and the electron beam which is emitted from the electron gun is deflected under the magnetic fields from deflection yokes (not shown) arranged on the neck of the CRT 203.
A slight variation in a mounting angle of the deflection yoke on the neck of the CRT 203 in a vertical direction causes a difference in a distortion wave form of a pincushion distortion, trapezoidal distortion and the like between the upper and lower areas of the screen, and hence a vertically asymmetrical distortion is generated.
Although the raster images at the upper and lower areas of the screen of the CRT have been corrected by the same amount so that the distortions are not readily visible, implementation of a feature to independently correct the side pincushion distortion at upper and lower areas of the screen has been demanded with a recent increase in a size and a planar structure of the screen of CRT.
Apparatuses for correcting the side pincushion distortion or vertically asymmetric distortion are disclosed in, for example, Japanese Patent Kokai Publication JP-A-6-334887, JP-A-5-308538 and JP-A-11-313222.
Among of them, in JP-A-6-334887 is disclosed a configuration of a corner pincushion distortion correcting wave generating circuit which is capable of independently correcting the distortions at corners in the upper and lower levels of the screen. As shown in FIG. 18, the correcting wave generating circuit comprises a lower corner part extracting differential comparator circuit 102 which receives a sawtooth wave signal 101 (vertical sawtooth wave signal) having a period of one vertical interval and extracts (slices) a part of the waveform corresponding to the lower area of the screen in the sawtooth wave signal 101; a multiplier circuit 105 for multiplying the extracted waveform; and an amplitude adjusting circuit (gain control circuit) 107 for adjusting the amplitude of the corrected wave, whereby the above-mentioned circuits correct the corner distortion in the lower area of the screen. The correction wave generating circuit further comprises an upper corner part extracting differential comparator circuit 103 which receives the sawtooth wave signal 101 and extracts a part of the waveform corresponding to the upper area of the screen in the sawtooth wave signal 101; a multiplier circuit 106 for multiplying the extracted waveform; and an amplitude adjusting circuit (gain control circuit) 108 for adjusting the amplitude of the corrected wave, whereby the circuits correct the corner distortion at the upper area of the screen. The correcting wave generating circuit further includes an amplitude adjusting circuit (gain control circuit) for adjusting the amplitude of the signal after the lower corrected wave is added to the upper corrected wave in an adder 109. A corner slice point bias circuit 104 provides reference bias voltages used in the lower and upper corner image extracting differential comparator circuits 102 and 103 respectively for extracting the lower and upper corner parts in images to the lower and upper corner extracting differential comparator circuits 102 and 103, respectively.
In JP-A-5-308538 is disclosed a left and right side pincushion distortion correcting apparatus for correcting the side pincushion distortion wherein a current flowing through the horizontal deflection coil of a horizontal output circuit is modulated in a vertical period by a parabolic voltage generated by integrating the vertical sawtooth wave voltage and wherein the apparatus comprises a correction section set up unit for forming a selection signal for each correction section which is obtained by dividing each of a vertical synchronization period with reference to the vertical synchronization signal or horizontal synchronization signal and a correction quantity adjusting unit which selects a correction adjusting voltage for each correction section in response to a selection signal of each correction section for superimposing the correction adjusting voltage of each correction section to the parabolic wave voltage. With this apparatus, the correction section is obtained by dividing the vertical synchronization period, the correction quantity adjusting voltage is given to each section, the correction quantity adjusting voltage in each correction section is put on the parabolic voltage, and parabolic wave voltage is generated at an integration circuit by integrating the vertical sawtooth wave voltage so that the central position of the amplitude of the vertical sawtooth wave matches the bottom of the parabolic wave voltage and the correction interval is preset with reference to the vertical synchronization signal and the correction section is set up with reference to the vertical synchronization signal.
In JP-A-11-313222, is disclosed an image distortion correcting apparatus which reduces distortion which is asymmetric in upper and lower parts of a screen. With this apparatus, an output of the side pin correcting circuit is a parabolic wave plus a third power wave (S-shape wave). The amplitude of the parabolic wave of the output signal is changed by varying a side pin control voltage, and hence a pincushion distortion which is symmetric in left and right sides of the screen can be corrected. The amplitude of the third power wave is changed by varying an S-shape distortion control voltage, and hence vertical S-shape distortion which is asymmetric in upper and lower parts of a screen can be corrected. The correction quantity in the third power wave is automatically changed depending upon the vertical size of the screen and vertical position by using a vertical sawtooth wave including information on the vertical size of the screen and the vertical position. With this apparatus, the distortion adjustment operation is complicated because the operation adjusts two signals (the parabolic wave and the 3rd power wave) for the distortion of the whole screen to be minimized.
As mentioned in the above, the apparatus which is disclosed in JP-A-6-334887 has two systems each comprising a differential comparator circuit, multiplier circuit and amplitude adjusting circuit for each of the upper and lower parts of a screen in order to control independently the upper and lower parts of the screen. Accordingly, the apparatus has a problem that the circuit scale increases. Parts in a vertical sawtooth wave corresponding to a lower part (lower corner) and an upper part (upper corner) of the screen are respectively extracted by the differential comparator circuits 102 and 103 and are respectively multiplied by the multiplier circuits 105 and 106. If there are variations in the offsets of two differential comparator circuits 102 and 103, the variations in the offsets are also multiplied by the multiplier circuits 105 and 106. The correction signal which is obtained by adding the lower waveform to the upper waveform may have an offset (step) due to the difference between the offsets in the upper and lower adjusting circuits. When the gains of two amplitude adjusting circuits 107 and 108 are changed, the offset voltage are changed, which may change the raster image size.
It is an object of the present invention to provide a distortion correcting circuit and a display apparatus in which a distortion is individually corrected for each one in the upper and lower parts of a screen while reducing a circuit scale.
Another object of the present invention is to provide a distortion correcting circuit and a display apparatus which is capable of correcting properly distortions in a lower and upper areas of a screen which are asymmetric even when a center of a vertical sawtooth wave does not correspond to a center of the screen.
At least one of the above mentioned objects is accomplished by a distortion correcting circuit in accordance with one aspect of the present invention which comprises: comparing means for comparing an amplitude of a sawtooth wave signal used for a vertical sweep (termed xe2x80x9cvertical sawtooth wavexe2x80x9d) with one predetermined reference level or a plurality of predetermined reference levels to detect which part of a screen in a vertical sweeping direction a currently sweeping position is located; amplifying means for adjusting an amplitude of a side pincushion distortion correction signal generated from said vertical sawtooth wave; and control means for individually setting a gain for each of partial regions of the screen to said amplifying means on the basis of a detection result by said comparing means.
In accordance with another aspect of the present invention is provided a distortion correcting circuit-comprising comparing means for comparing an amplitude of a vertical sawtooth wave with a predetermined reference level to detect which of an upper and lower areas of a screen a current sweeping position is located at; and control means for individually setting for the upper and lower areas of the screen the gain of amplifying means which amplifies the amplitude of a side pincushion distortion correction signal generated from said vertical sawtooth wave based upon a result of the detection conducted by said comparing means.
More specifically, a distortion correcting circuit in accordance with another aspect of the present invention comprises: a comparator circuit which receives a sawtooth wave signal used for a vertical sweep (termed xe2x80x9cvertical sawtooth wavexe2x80x9d) and compares an amplitude of said vertical sawtooth wave in a scanning period with one reference level or a plurality of different reference levels, each of which is set respectively to an amplitude value of the vertical sawtooth wave corresponding to each predetermined position of the screen in a vertical sweeping direction; a variable gain type amplifier circuit which adjusts an amplitude of an output signal from a multiplier circuit for generating a parabolic wave; and a control circuit which alternatively selects one of a plurality of gains to provide the selected gain to said variable gain type amplifier circuit on the basis of a comparison result by said comparator circuit depending upon which of regions divided by said reference level the amplitude of said vertical sawtooth wave is located, wherein a distortion correction signal is output from said variable gain type amplifier circuit.
A distortion correcting circuit in accordance with another aspect of the present invention comprises: a comparator circuit which receives a sawtooth wave signal used for a vertical sweep (termed xe2x80x9cvertical sawtooth wavexe2x80x9d) and compares the voltage of said vertical sawtooth wave in a scanning period with a reference level or a plurality of reference levels, each of which is set to an amplitude of the vertical sawtooth wave corresponding to a predetermined vertical position of a screen; a variable gain type amplifier circuit which amplifies said vertical sawtooth wave; a control circuit which selects one of plurality of predetermined gains depending upon which of regions divided by said reference level the amplitude of said vertical sawtooth wave is located on the basis of a comparison result by said comparator circuit and provides the gain selected to the variable gain type amplifier circuit; and a multiplier circuit which generates a parabolic wave from an output signal output from said variable gain type amplifier circuit to output said parabolic wave as a distortion correction signal
Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated of carrying out this invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as restrictive.